International patent publication, WO 2012/107331 discloses a device for monitoring a laser cutting process which may serve for registering characteristic variables of a laser cutting process, for example an imminent loss of cut. WO 2012/107331 notes that an imminent loss of cut is identified when a predetermined gap width of the cut gap is undershot. Alternatively or additionally, the area of the observed cutting front is compared to a reference area that corresponds to the area of the cutting front in the case of a good cut or a quality cut. A loss of cut can also be detected if the radiation intensity emitted by the reference area exceeds a threshold for the target brightness in the case of a normal cut.
WO 2013/053832 A1 describes a device that measures backscattered light during a laser cutting process to verify the cut quality. The measured intensity of the backscattered light is lower if the cut extends through the workpiece. To optimize a removal of slag, a frequency or a pressure of gas pulses used during the cutting process are adapted by means of a control apparatus in such a way that the measured intensity of the backscattered light assumes a minimum value.
The general cause of a loss of cut lies in an insufficient energy influx into the workpiece. An energy input per unit length that is too low leads to flattening of the cutting front, i.e., to an increase in the cutting front angle. As a result of the increase in the cutting front angle, the molten material at the lower edge of the cut can no longer be completely driven away and the latter solidifies in the kerf. A closure of the cut lower edge leads to process irregularities that generally permanently prevent a severance cut. Therefore, the cutting front angle, which represents a characteristic variable of the cut gap, is an indicator for an imminent loss of cut.
In principle, the cutting front or the cutting front angle can be rendered measurable by a vertical observation of the temperature radiation emanating from the interaction region in a coaxial manner with the high-power beam. In the case of a known workpiece thickness, the cutting front angle can be deduced on the basis of the length of the emitting region in the kerf. A problem here is that a dripping melt filament below the workpiece leads to an elongation of the emitting region, and so it is not generally possible to measure the cutting front angle reliably in this manner.
WO 2012/107331 A1 proposes the detection of a cutting front upper edge and a cutting front lower edge as material boundaries of the workpiece and the determination of the cutting front angle of the laser cutting process therefrom, taking into account the thickness of the workpiece. To this end, the distance between the cutting front upper edge and the cutting front lower edge is typically measured along the gap center of the cut gap or kerf in the visible wavelength range. If the cutting front angle deviates from a setpoint value or setpoint range, this can indicate a cutting error or a non-ideal work point, which can be corrected by suitable measures, e.g., by adapting the cutting speed.
When observing the process coaxially through the cutting nozzle, a problem may arise in that the observation region is delimited by the generally circular inner contour of the cutting nozzle, both when observing the temperature radiation, the backscattered high-energy radiation and when observing material boundaries. In the case of flame cutting processes in particular, use is made of small nozzle diameters, and so the cutting front lower edge lies outside of the observation region restricted by the nozzle opening, even in the case of a good cut, and this can impede reliably determining the cutting front angle.
DE 10 2011 016 519 A1 describes a method and a device for controlling the processing of a workpiece by means of a high-energy processing beam, wherein the processing beam passes through a lens that can be moved perpendicular to the optical axis thereof in order to displace an impact point of the processing beam on the workpiece. In one example, provision is made of a monitoring camera for generating an electronically evaluable image, the imaging beam path of the camera is focused on the impact point by the lens.
WO 2012/107331 A1 also discloses the practice of deducing the presence or lack of a burr formation at the cut gap on the basis of the image of the interaction region. By way of example, by virtue of the lack of a recurring variation in the intensity of the thermal image in the region of the cut gap and/or in the case of an occurrence of three luminous strips emanating from the cutting front, it is possible to deduce the presence of a burr formation in the case of a fusion cutting process. In the case of a constructional steel flame-cutting process (using oxygen as cutting gas), periodically recurring grooves or furrows can be detected in the thermal image or in the visible wavelength range at the cut edges of the kerf and an imminent material burn-up can be deduced on the basis of the frequency of the furrows.
The theme of the dissertation “Überwachung, Regelung and Automatisierung beim Hochgeschwindigkeitsschneiden von Elektroblechen mit Laserstrahlen” by Frank Schneider, Shaker Verlag, 2005, is high-speed cutting with cutting or advance speeds of up to 100 m/min. In this application, a melt accumulation may form behind the cutting front and it may become so large in the case of a high speed that the capillary between the melt and the cutting front is temporarily closed off by the downward melt flow. In this case, the laser radiation is reflected back from the blocked capillary and measured by a pyro-detector or a thermopile with a dragging observation or an observation with a time lag. In the case of a dragging observation and high speeds, there are intense, short reflections and therefore a greatly fluctuating measured signal. Since the standard deviation of the measured signal typically increases with increasing speed, the high-speed cutting process can be regulated on the basis of the standard deviation.